The HI segment on the EDI claim is where the Diagnosis Codes are sent. The payer is indicating that one or more of the diagnosis codes you have entered is not valid. This could mean that it is not from the data set of diagnosis codes (ICD) or it could mean that a diagnosis code you supplied is not accepted by this payer.
How to Fix
The diagnosis codes are entered / edited from the add / edit client form, in the section labeled Cases. Open up the edit client form for this page. At the bottom right, click the edit icon associated with the diagnosis code set. Remove the offending diagnosis code. Remember to save the client when done.